(1) Field of the Invention
The present invention relates to a manufacturing method of an arc tube composed of a glass tube having a spirally wound portion of which tube axis is contained within one plane. The present invention also relates to the arc tube, a low-pressure mercury lamp, and an illumination apparatus.
(2) Description of the Related Art
Low-pressure mercury lamps for general purpose illumination includes fluorescent lamps composed of a circular arc tube (hereinafter, “circular fluorescent lamp”). One feature of circular fluorescent lamps lies in the flatness in shape.
Recently, there has been a growing demand for downsizing circular fluorescent lamps. This is because the downsizing of a circular fluorescent lamp leads to downsizing of an illumination apparatus for use with the fluorescent lamp attached thereto.
One example of an arc tube that is smaller than conventional circular arc tubes is an arc tube composed of a glass tube that spirally winds toward an end thereof, around an imaginary axis farther and farther from the imaginary axis. The tube axis of the wound portion of the glass tube is contained within one plane. This shape is referred to as a “flat spiral”, because the axis of the glass tube is contained within one plane. (See JP Patent Application Publication No. 09-92154.)
With the glass tube having a flat spiral shape, the space enclosed within the circular arc tube is effectively utilized. Thus, the arc tube may be made smaller in outermost diameter, without shortening the discharge distance in comparison with that of the conventional arc tube.
The arc tube having a flat spiral shape is formed through a molding step and a deforming step as follows. In the molding step, a softened glass tube is wound downwardly around the surface of a circular cone (this surface is referred to as an “imaginary conical surface”), and thus the glass tube winds farther and farther away from the axis of the imaginary conical surface (this axis is referred to as “imaginary axis”). This shape is referred to as a “stereo spiral”, because the axis of the glass tube shifts in the direction of the imaginary axis as the grass tube spirals. In the deforming step, the spirally wound glass tube is heated throughout its length, so that the glass tube deforms under its own weight.
Unfortunately, however, there is a problem in the conventional method of manufacturing an arc tube into a flat spiral shape. According to the conventional method, upon flattening the wound portion of the glass tube in the direction of the imaginary axis, the glass tube is heated to its softening point in order to allow the glass tube to deform under its own weight. As a result, the wound portion of the glass tube fails to maintain its tubular shape. For this reason, it is often the case where the glass tube after the deforming is distorted in cross section, adjacent turns of the glass tube come into contact with each other, and the end portion of the glass tube tapers. In short, according to the conventional method, it is difficult to manufacture an arc tube in excellent design.